map_if: Apply a function to each element of a vector conditionally

In purrr: Functional Programming Tools

Apply a function to each element of a vector conditionally

Description

The functions map_if() and map_at() take .x as input, apply the function .f to some of the elements of .x, and return a list of the same length as the input.

• map_if() takes a predicate function .p as input to determine which elements of .x are transformed with .f.

• map_at() takes a vector of names or positions .at to specify which elements of .x are transformed with .f.

Usage

map_if(.x, .p, .f, ..., .else = NULL)

map_at(.x, .at, .f, ..., .progress = FALSE)

Arguments

.x	A list or atomic vector.
.p	A single predicate function, a formula describing such a predicate function, or a logical vector of the same length as .x. Alternatively, if the elements of .x are themselves lists of objects, a string indicating the name of a logical element in the inner lists. Only those elements where .p evaluates to TRUE will be modified.
.f	A function, specified in one of the following ways: A named function, e.g. mean. An anonymous function, e.g. ⁠\(x) x + 1⁠ or function(x) x + 1. A formula, e.g. ~ .x + 1. Use .x to refer to the first argument. No longer recommended. A string, integer, or list, e.g. "idx", 1, or list("idx", 1) which are shorthand for ⁠\(x) pluck(x, "idx")⁠, ⁠\(x) pluck(x, 1)⁠, and ⁠\(x) pluck(x, "idx", 1)⁠ respectively. Optionally supply .default to set a default value if the indexed element is NULL or does not exist. Wrap a function with in_parallel() to declare that it should be performed in parallel. See in_parallel() for more details. Use of ... is not permitted in this context.
...	Additional arguments passed on to the mapped function. We now generally recommend against using ... to pass additional (constant) arguments to .f. Instead use a shorthand anonymous function: # Instead of x |> map(f, 1, 2, collapse = ",") # do: x |> map(\(x) f(x, 1, 2, collapse = ",")) This makes it easier to understand which arguments belong to which function and will tend to yield better error messages.
.else	A function applied to elements of .x for which .p returns FALSE.
.at	A logical, integer, or character vector giving the elements to select. Alternatively, a function that takes a vector of names, and returns a logical, integer, or character vector of elements to select. : if the tidyselect package is installed, you can use vars() and tidyselect helpers to select elements.
.progress	Whether to show a progress bar. Use TRUE to turn on a basic progress bar, use a string to give it a name, or see progress_bars for more details.

See Also

Other map variants: imap(), lmap(), map(), map2(), map_depth(), modify(), pmap()

Examples

# Use a predicate function to decide whether to map a function:
iris |> map_if(is.factor, as.character) |> str()

# Specify an alternative with the `.else` argument:
iris |> map_if(is.factor, as.character, .else = as.integer) |> str()

# Use numeric vector of positions select elements to change:
iris |> map_at(c(4, 5), is.numeric) |> str()

# Use vector of names to specify which elements to change:
iris |> map_at("Species", toupper) |> str()

purrr documentation built on Nov. 5, 2025, 7:27 p.m.